The present invention relates to operation of an electro-optic modulator, and more particularly to operation of a Mach-Zehnder Interferometer (MZI) type lithium niobate (LiNbO3) modulator.
Designers and manufacturers of data communications systems are under constant pressure to increase the data rate of their systems. One type of high-performance data communications system uses fiber-optic materials to transmit data over long distances at high speed in the form of modulated electromagnetic waves.
A popular method of modulating the electromagnetic waves employs a Mach-Zehnder Interferometer (MZI) type modulator. The operation of the MZI-type modulator is based on constructive/destructive interference among two optical paths. The optical paths are defined by an optical waveguide, known as a coplanar waveguide (CPW), implanted in a substrate housing the modulator. The substrate may be formed from lithium niobate (LiNbO3). The LiNbO3 modulator modulates the output optical signal by varying a phase difference between the electromagnetic waves traversing the two optical paths from 0 and 180 degrees, which is accomplished by applying a drive voltage (Vxcfx80) across electrodes of the modulator substrate. Application of the drive voltage (Vxcfx80) effects a change in optical refractive index of the optical paths of the optical waveguide via electrical fields generated in the substrate, thus altering the phase of lightwaves traversing the optical paths. The resulting change in phase of the lightwaves traversing the optical paths result in constructive/destructive interference patterns appearing in the recombined output optical signal, thereby permitting modulation of the output optical signal between two states (e.g., logical xe2x80x9c1xe2x80x9d and xe2x80x9c0xe2x80x9d).
One problem in further increasing the attainable data communication rate is an attendant increase in drive voltage. High frequency conductor losses and phase velocity mismatch between electrical and optical signals tend to reduce the attainable bandwidth of the modulator. These issues may be resolved by reducing the total length L along which the electrode is coupled to the optical waveguide. Because the product Vxcfx80, the swing or drive voltage, times L is constant for a given modulator design, a reduction in L must be accompanied by a consequent increase in Vxcfx80. However, increased drive voltage Vxcfx80 may be impractical due to use of high-speed integrated circuits (ICs) to supply power to the modulator, since high-speed ICs are characterized by reduced transistor output breakdown voltages. Thus, increasing Vxcfx80 is undesired.
One approach to solving the drive voltage problem uses a push-pull arrangement for the modulator electrodes. FIG. 1 illustrates such a push-pull arrangement 20, in which modulation may be achieved by applying approximately half of the usual drive voltage Vxcfx80. Referring to FIG. 1, two arms 52, 54 of an MZI-type modulator 56 are coupled to a controller circuit which drives the arms 52, 54 with complementary voltages (opposing polarity). For example, data voltage source 24 applies complementary voltages V1 and V2 to arms 52, 54 respectively. The data voltage source 24 is coupled to the arms 52, 54, and applies drive voltages V1, V2 of equal amplitude and opposite polarity to the arms 52, 54 according to an applied data source signal input 22. The terminating resistors 44, 46 shown in FIG. 1 are not necessarily required and are included only to illustrate conventional termination well known in the art.
FIG. 3 illustrates a cross sectional view IIIxe2x80x94III (FIG. 1) showing electrical fields 114 generated in the vicinity of the arms 52, 54 and the optical waveguide 112 for this type of arrangement of the MZI-type modulator 56. It should be noted that the electrical fields 114 are shown for a z-cut crystal orientation of the substrate. For an x-cut crystal orientation of the substrate, the electrical fields 114 may be oriented differently.
For drive voltages V1 and V2 of opposing polarity, the final drive voltage requirements of the modulator is Vxcfx80/2. Thus, with the push-pull arrangement of FIG. 1, drive voltage requirements for the modulator are reduced approximately by a factor of 2.
Another problem affecting the development of high-speed modulators is adjustable chirp factor, due to the need to apply varying drive voltages to the arms of the modulator. Adjustable chirp modulators offer the ability to increase the transmission span of a data communications system employing optical fiber with particular dispersion properties. For a push-pull arrangement similar to that illustrated in FIG. 1, chirp may be adjusted by varying the relative amplitudes of the voltages applied to each arm 52, 54 of the MZI-type modulator 56.
FIG. 2 illustrates a modulator arrangement 40 in which the controller circuit includes first and second variable data voltage sources 76, 86. The push-pull modulator shown in FIG. 2 can produce chirp factors varying between xe2x88x921 and +1, depending on the relative amplitudes of the drive voltages applied to the arms 52, 54. For example, for a chirp factor of zero (0), equal amplitude (but opposite polarity) swing voltages of Vxcfx80/2 may be used for V1 and V2, as shown in FIG. 4a. Chirp factor may be denoted by xcex1, as shown in FIGS. 4a-4c. For a chirp factor of one (1), one of the arms, e.g., 52, may be driven with an amplitude of zero volt (V1=0), while the other arm, e.g., 54, may be driven with a swing voltage V2 of Vxcfx80 volts, as shown in FIG. 4b. For a chirp factor of negative one (xe2x88x921), one of the arms, e.g., 52 may be driven with a swing voltage V1 of Vxcfx80 volts, while the other arm, e.g., 54, may be driven at a voltage V2 of zero volts, as shown in FIG. 4c. The first and second variable data voltage sources thus may vary the amplitudes of V1 and V2, to effect a variation in chirp factor, while maintaining their sum to keep Vxcfx80 constant. Note that for adjustable chirp, the variable data voltage sources 76, 86 each must be independently variable up to a swing voltage of Vxcfx80 volts.
Accordingly, there is a strong desire and need to develop an adjustable chirp modulator having reduced drive voltage requirements.
An electro-optic modulator and associated method are provided for an adjustable chirp arrangement exhibiting reduced drive voltage requirements. This and other advantages are achieved using an optical waveguide comprised of two pairs of coplanar modulation strips (CPS), the strips being driven by a controller circuit which applies voltages of equal amplitude and opposite polarity to the strips of each pair. The voltage applied to a first pair of strips may differ from the voltage applied to a second pair of strips, so that the chirp may be adjusted if desired.
The electro-optic modulator includes first and second pairs of coplanar modulation strips and a controller circuit including first and second drive circuits respectively coupled to the first and second pairs of coplanar modulation strips, wherein the first drive circuit drives the first pair using first and second differential signals, and the second drive circuit drives the second pair using third and fourth differential signals. The differential voltages are of equal amplitude and opposite polarity and reduce the required drive voltage by approximately a factor of two.
In another aspect of the invention, the first and second drive circuits include respective first and second variable data voltage sources. A chirp factor of the modulator may be adjusted by varying the output voltage of the respective first and second variable data voltage sources between approximately 0 volt and one-half of a final drive voltage of the modulator.